1. Field of the Invention
This invention relates to methods of isomerizing the stereoisomers of cycloaliphatic diamines and is more particularly concerned with producing di-(p-aminocyclohexyl)methane rich in trans,trans-isomer.
2. Description of the Prior Art
Di-(p-aminocyclohexyl)methane is generally prepared by the catalytic hydrogenation of di-(p-aminophenyl)methane; see, for example, U.S. Pat. Nos. 2,494,563; 2,606,924; 2,606,928; 3,591,635 and 3,856,862. The products so obtained are found to be mixtures of the various possible stereoisomers, namely, the cis,cis-, cis,trans- and trans,trans-isomers.
For many purposes it is desirable to employ the substantially pure trans,trans-isomer. For example, polyamides derived from the latter isomer and dibasic aliphatic and aromatic carboxylic acids produce synthetic fibers having highly desirable properties; see, for example, Tedder et al., Basic Organic Chemistry, Part 5, pp. 283-4, John Wiley and Sons, London, 1975. Various methods have been reported for the separation of the stereoisomers one from another but it is obviously much more desirable from an economic standpoint to be able to convert the cis,cis- and cis,trans-isomers to the trans,trans-isomer. Such isomerization has been accomplished hitherto by subjecting the mixture of isomers to high temperature in the presence of catalysts; see, for example, Japan 71/30,835, Japan 71/16,979, German No. 1,768,427 and German Offen. 2,301,106. This method, in addition to the disadvantages of using high temperatures, results in significant overall loss of the diamine due to side reactions.
I have now found that the above isomerization can be effected readily, in substantially quantitative yield and without the use of elevated temperatures, by the process which is described below.